Next we apply the change of variables (\(\alpha \)  parameters)
using the trick from [1]}:
\(\alpha _1 = \eta _1 (1-\xi _1)\,, \qquad \alpha _2 = \eta _2 (1-\xi _2)\,, \qquad \alpha _3 = \eta _1 \xi _1\,, \qquad \alpha _4 = \eta _2 \xi _2\,.\) 
